The following invention pertains to the art of manually providing positive-pressure artificial ventilation to the non-breathing patient, and particularly to patients suffering acute respiratory and/or cardiopulmonary arrest.
The critical role of the lungs in maintaining life is well known—most individuals are aware of the body's need for oxygen to maintain cellular metabolism and recognize the critical role of the lungs in providing the necessary amounts of oxygen to support life. When breathing stops, as it does during cardiac arrest, a vicious circle of events take place as the cells of the body attempt to survive without oxygen necessary for metabolism. These events constitute the initial phase of biological death, which rapidly progresses without prompt emergency treatment.
Accordingly, one of the first priorities in resuscitation is to establish a means to provide artificial ventilation to a patient. This begins with what is known in the art as establishing an airway, which is the process of providing an open passage for air to travel through the patient's mouth, throat, and trachea (or windpipe) to the lungs. This can initially be achieved by properly positioning the patient's head and neck.
Once an airway is established, one next provides artificial ventilation by forcing air into the patient's mouth, through the trachea, and into the lungs. When utilizing the mouth-to-mouth method of ventilation, no ventilatory device is used. This is the form of artificial breathing taught by the American Heart Association and the European Resuscitation Council as part of Basic Life Support (BLS) courses on cardiopulmonary resuscitation (CPR). However, healthcare professionals, both in-hospital (physicians, respiratory therapists, nurses, etc.) and pre-hospital (emergency medical technicians, paramedics) most often use various medical devices to provide artificial ventilation. These can include pocket masks (through which the rescuer manually blows to inflate the patient's lungs), demand valves (a mechanical device which inflates the patient's lungs with compressed oxygen when a button is manually pressed on the face mask), and automatic transport ventilators (fully automatic mechanical ventilators which deliver sequential breaths to the patient), however the device which is most frequently employed is the manual resuscitator.
Also known as an “Ambu bag” or “bag-vale-mask” (BVM), the manual resuscitator is a balloon-type device frequently portrayed on fictional or documentary medical television programs. Essentially a hand-powered air pump, the device consists of a squeezable, randomly pliable self-inflating bag (or fluid chamber) which, when squeezed by the operator, displaces air from the bag and out a port to which a face mask can be connected. More specifically, the technique to use a manual resuscitator comprises utilizing one hand to perform the combined task of maintaining proper positioning of the head (to maintain the airway), while applying pressure on the mask to form a seal between the patient's face and the mask. At the same time, the operator uses the free hand to squeeze the fluid chamber and thus displace air into the patient's lungs under positive pressure. When the fluid chamber is released the patient passively exhales, while the fluid chamber returns to its natural, inflated state in preparation for the next breath.
The manual resuscitator was originally designed in the 1930 's, and over the years has fortified its position as the first-line device employed in artificial ventilation. The balloon-type design is inherently intuitive—little training is required to learn how to operate the device, and its simplicity makes it ideal for use in frantic environments commonly associated with resuscitation efforts. Its simple design also makes it inherently reliable—another highly desirable attribute for a medical device essential for resuscitation. Finally, the cost of manual resuscitators is very low, making it possible for them to be stored in readiness in highly accessible places throughout the hospital.
With these strong attributes intrinsic to the basic design of the manual resuscitator, little has been done over the years to modify it. Improvements have included utilization of new materials in the construction of the squeezable fluid chamber to facilitate a better, more comfortable grip. Other design refinements have decreased weight and decreased manufacturing cost, further contributing to the economy of disposable versions of the device.
Particularly during the past two decades, however, a number of advancements in medicine have greatly contributed to the understanding of pulmonary physiology and the pathophysiology of cardiorespiratory arrest. These advancements inspired additional clinical studies to specifically assess the performance of manual resuscitators, which have since been proven to have grave inadequacies.
One of the first problems identified was a general inability for single rescuers to simultaneously use one hand to maintain the face mask seal, use the other hand to squeeze the chamber, and generate a breath of sufficient volume (called the tidal volume). One reason for these low volumes is it is difficult to maintain an airtight seal between the face mask and the patient's face with one hand, resulting in frequent loss of a significant portion of the tidal volume to leakage. Another problem contributing to small tidal volumes is the randomly pliable nature of the skin of the fluid chamber of the device. When squeezed, areas not in direct contact with the rescuer's hand bulge out, reducing the efficiency of the manual compressing action which constitutes operation of the device. Other studies proved these deficiencies were not related to the level of training of the rescuer—paramedics, nurses, and physicians operating the device were all found to be unable to consistently provide ventilation at recommended levels. While it may seem obvious this problem can be overcome simply by providing more frequent breaths, this strategy can actually result in further decreased ventilation to the patient.
This apparent paradox, where increasing ventilatory rate may actually lead to decreased overall ventilation of the lungs, is related to a physiologic principle known as anatomic deadspace. The actual exchange of gases in the lungs, called respiration, occurs in tiny air sacks which are surrounded by a web of blood capillaries. These sacks, called alveoli, is where the blood receives oxygen from the air inhaled in exchange for carbon dioxide waste, which is exhaled. In every individual, a significant portion of a given breath remains in the mouth, throat, trachea, and the various distal airways in the lungs, which are collectively referred to as deadspace. Residual air occupying deadspace at the end of inhalation never actually reach the alveoli and therefore do not contribute to gas exchange. Accordingly, since deadspace is an anatomic constant unaffected by the size of the breath administered, when small breaths are given deadspace negates 25-35% the of total tidal volume delivered, whereas if large breaths are given deadspace consumes only 10-20% of each tidal volume. Consequently, one ventilating rapidly but with small tidal volumes is likely to deliver less effective ventilation than one would by utilizing a larger tidal volume at a slower rate.
This paradox has significant clinical implications. Frequently during resuscitation, certain blood tests are performed which measures the amount of oxygen and carbon dioxide in the blood. When such examinations reveal decreased oxygen levels or, more importantly, elevated amounts of carbon dioxide in the blood, the individual ventilating is usually prompted to provide increase their efforts. The natural response would be to increase the ventilatory rate, however, higher ventilatory rates have been associated with increased operator hand fatigue and inattentiveness. Consequently, tidal volumes have been observed to decrease as ventilatory rates increase. Therefore, despite increased ventilatory rate (and operator impression they are providing improved ventilation), overall ventilatory effectiveness may actually decrease, because as tidal volume decreases anatomic deadspace represents increasing proportions of each breath, which can provide a greater negative affect on alveolar ventilation than the positive effect of a higher rate.
This concept is not universally recognized among health care providers. As a result, many continue to inappropriately regard the effectiveness of the manual resuscitator as rate-dependent rather than volume-dependent.
Pursuant to findings demonstrating inability of single rescuers to generate adequate volumes, authoritative agencies recommended implementation of a two-person technique to utilize manual resuscitators—one-person to maintain a face mask seal with two hands, while the other rescuer squeezes the fluid chamber using two hands. Clinical studies performed thereafter sought to document delivery of higher tidal volumes consistent with resuscitation standards.
While increased volumes are produced by the two-person technique, clinical studies also identified significant hazards associated with the two-person technique. To compensate for the aforementioned bulging-out phenomenon during the one-handed technique, resuscitator manufacturers make fluid chambers disproportionally large. Thus when two hands are used to provide a breath, improved surface-area contact between the hands and the fluid chamber decrease the extent of outward bulging, resulting in the generation of excessive volumes, air flow rates, and airway pressures.
Generation of excessive volumes, pressures, and flow rates has been shown to cause significant hazards to the patient. One study in particular assessed the distribution of gas between the lungs and stomach in patients ventilated with manual resuscitators. Even with the one-person technique, air flow rates and airway pressures were excessive enough to cause air to preferentially enter the stomach, and at times, flow to the stomach actually was greater than the amount received by the lungs. Inflation of the stomach with air (called gastric insufflation) markedly increases the risk of patient vomiting, potentially resulting in stomach contents entering the lungs (a grave complication). In fact, the danger and incidence of gastric distention associated with the use of prior art manual resuscitators has recently been determined to be great enough to recommend utilization of child-size versions of the prior art on adult patients, since the smaller size of the child device provides a safeguard against generation of excessive volumes, pressures, and flow rates which leads to a decreased incidence of this complication. Accordingly, some resuscitation authorities now recommended that, in lieu of a truly safe and effective ventilatory adjunct, when ventilating an adult with the prior art it is preferable to compromise ventilatory effectiveness in order to achieve greater security from complications associated with adult-versions of the device.
Other measures can be employed to palliate these deficiencies of the prior art. A more definitive form of airway control involves placement of a tube (called an endotracheal tube) directly into the patient's trachea, thus isolating the airway from the gastrointestional tract. After intubation, the face mask can be detached and the manual resuscitator directly connected to a port on the endotracheal tube. This obviates the need for active airway maintenance, provides definitive airway protection, and allows a single rescuer to use two hands to ventilate the patient.
However, endotracheal intubation is a medical procedure requiring considerable skill and experience and is usually performed by a physician, respiratory therapist, or nurse anesthetist. When performed successfully on the first attempt intubation can be completed in less than 10 seconds; however, the procedure is often successful only after multiple attempts, spanned over several minutes. Indeed, at times intubation attempts are all unsuccessful on a particular patient, perhaps due to trauma, anatomic aberrancies, and/or inexperience of the individual attempting to perform the procedure. Until the patient is successfully intubated, the aforementioned deficiencies of the prior art continue to jeopardize patient survival.
Even after successful intubation, additional studies have shown use of prior art manual resuscitators are still associated with significant risks. As previously indicated, one-handed operation results in inadequate ventilation to the patient, while two-handed operation is associated with excessive volumes, flow rates, and pressures.
One-handed operation in the intubated patient continues to result in ventilatory volumes which decrease the ability of the lungs to provide oxygen to the blood. More importantly, this also affects the amount of carbon dioxide waste the lungs can remove from the blood, which is also an absolute requirement for the sustainment of life. Carbon dioxide is created as a by-product of metabolism, and the accumulation of excessive amounts in the blood, called hypercarbia, causes an acidic pH of the blood which has various negative affects on the body. Research has definitively shown hypercarbia has several potent effects on the heart which directly contributes to decreased patient survival from cardiac arrest. Hypercarbia has been shown to increase the tendency of the heart to degenerate into a chaotic arrhythmia called ventricular fibrillation, where the heart muscle essentially quivers and produces no pumping action. Additionally hypercarbia has been demonstrated to support sustainment of ventricular fibrillation, and cause it to recur after a normal rhythm has been successfully restored. Finally, significant decreases in the effectiveness of electrical defibrillation (the treatment for ventricular fibrillation) has proven to be directly related to the presence of hypercarbia. Even in normal, beating tissue, hypercarbia has been demonstrated to immediately decrease the pumping strength of heart muscle. The only way to control hypercarbia, and thus oppose these potent effects on the heart, is to provide effective ventilation to the patient. Accordingly, since one-handed operation has been shown to generate inadequate volumes to both intubated and unintubated patients, it would appear its employment during resuscitation may contribute to patient mortality.
However, employing a two-handed technique in the intubated patient, while preventing the harmful effects of hypoventilation and hypercarbia, are associated with the aforementioned risks due to generation of excessive tidal volumes, airway pressures, and flow rates other significant risks. The potential for lung injury is more pronounced when the patient is intubated since, under these circumstances, the endotracheal tube provides a sealed, direct connection between the manual resuscitator and the patient's lungs. Accordingly, excessive or over-aggressive ventilatory techniques, encouraged in-part by the intense environment of frantic resuscitation efforts, have been documented to cause traumatic injury to the lungs, particularly in pediatric and elderly patients.
In patients who are successfully resuscitated, high inflation pressures have been shown to be a contributing factor to the development of the Acute Respiratory Distress Syndrome (ARDS), the treatment of which requires sustained mechanical ventilation and intensive-care hospitalization over several weeks or months. Treatment of ARDS is extensive, extremely costly, and frequently unsuccessful. Excessive ventilatory volumes and pressures can also cause acute life-threatening lung injury through actual disruption of lung tissue. In addition to pneumothorax (collapsed lung), there have been several reports of cases where high inflation pressures generated by manual resuscitators have caused air to directly enter the bloodstream (called an air embolism), a complication which is almost invariably fatal. Even in patients with a perfusing rhythm, high airway pressures are known to decrease blood pressure, cardiac output, and oxygen delivery significantly by causing the lungs, inflated with high pressures, to compress the heart and the large blood vessels in the chest.
Another deficiency of the prior art is related to high variability of tidal volumes generated by the device. Small changes in hand position on the fluid chamber are exaggerated by the bulging-out effect, causing disproportionate changes in tidal volumes. This attribute contributes to the inability of the prior art device to provide consistent and predictable ventilation to the patient, regardless of operator technique. This has been postulated to interfere with the ability to interpret certain blood tests which assess the effectiveness of ventilation and which are fundamental measures of ventilation effectiveness.
Another disadvantage associated with breath-to-breath inconsistency is an inability to detect certain life-threatening conditions which are associated with increasing lung resistance. The presence of a life-threatening lung injury (e.g., tension pneumothorax) may be detected early by noticing a requirement for progressively increasing ventilatory pressures to achieve delivery of a specific tidal volume. With marked breath-to-breath variation associated with the prior art, this condition is likely to be apparent only after advanced progression of the injury begins to contribute to circulatory collapse and appearance of other ominous physical findings. Late identification of such underlying injuries further jeopardizes the patient and complicates treatment.
Another challenge in resuscitation is monitoring placement of the endotracheal tube. The trachea, after it descends from the throat, bifurcates into two main branches each of which go to one of the two lungs. The first concern with the placement of the endotracheal tube is to be sure it has been positioned in the trachea instead of the esophagus (which leads to the stomach). While several techniques to detect carbon dioxide (which is not present in quantity in the stomach) provides assurance the tube has been placed in the trachea, aside from interrupting resuscitation to obtain a chest X-ray there is no way to readily assess exact placement of the endotracheal tube within the trachea. This is a significant concern; if the tube is advanced or displaced too far, the end of the tube may progress beyond the bifurcation and thus only one lung will be ventilated (probably resulting in patient death if not recognized). Due to the aforementioned breath-to-breath variability of the prior art, this condition, also marked by increased pressures associated with the delivery of a specific volume, is not likely to be obvious via the operation of the prior art.
Accordingly, one can see the prior art manual resuscitator, while simple and inexpensive in design and operation, has multiple barriers to the consistent delivery of safe and effective artificial ventilation. Its use in unintubated patients either results in inadequate ventilation and hypercarbia with the one-person technique (or if a child-size device is employed), or the unacceptable risk of gastric insufflation and aspiration of vomitus associated with the two-person technique with the full-sized adult device. When used on intubated patients employment of a one-handed technique contributes to hypoventilation and hypercarbia, the latter of which has been proven to have several potent affects on the heart which directly contribute to increased mortality from cardiac arrest. When two-handed operation is used on intubated patients, the lack of an ability to guard against the generation of excessive airway pressures and volumes has been demonstrated to result in circulatory depression and a significant incidence of lung injury, the latter of which results in further complications, increased hospitalization, and/or death. Furthermore, breath-to-breath variability associated with the prior art results in unpredictable and inconsistent ventilation (affecting the ability to interpret certain blood tests), and decreases or inhibits the sensitivity for one to detect progressively increasing pulmonary resistance to ventilation, which can be indicative of endotracheal tube displacement or the presence of underlying life-threatening intrathoracic injury.
Thus, it can be seen there is a need for a device which shares beneficial attributes of the prior art (simplicity, reliability, affordability, and disposability) while offering new capabilities which address the several performance inadequacies which have been proven through clinical studies. There is a need for a device which can guard against the hazards associated with hypoventilation and hypercarbia by consistently providing predictable volumes of air without regard for hand placement, the size of a rescuer's hands, or the number of hands employed for operation. There is a need for a device which can provide safeguards against the generation of excessive volumes and airway pressures which have been demonstrated to contribute to gastric insufflation, circulatory depression, complications, extended hospitalization, and fatal lung injury. Finally, there is a need for a device which can decrease breath-to-breath variability in volume to facilitate a predictable level of artificial ventilatory support, enable more accurate interpretation of blood test results, and contribute to early identification of decreased lung compliance secondary to life-threatening intrathoracic injury or clinically significant displacement of an endotracheal tube.